1. Technical Field
The present invention relates generally to semiconductor lasers, and more particularly to improving the performance of a tunable Vertical Cavity Surface Emitting Laser (VCSEL). The invention describes an approach adopted to minimize the device capacitance and therefore improve the device high frequency performance and impedance matching.
2. Background Art
Optical communication systems are a substantial and fast growing constituent of communications networks. Such optical systems include, but are not limited to, telecommunication systems, cable television systems, and Local Area Networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical systems are configured to carry an optical channel of a single wavelength over one or more optical wave-guides. To convey the information form plural sources, time division multiplexing is frequently employed (TDM). In time division multiplexing, a particular time slot is assigned to each information source, the complete signal being constructed from the signal collected from each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and the need to generate high peak power pulses.
While the need for communication systems increases, the current capacity of existing wave-guiding media is limited. Although capacity may be expanded, e.g. by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of the existing optical wave-guides.
Wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) have been explored as approaches for increasing the capacity of the existing fiber optic networks. Such system employs plural optical signal channels, each channel being assigned a particular channel wavelength. In a typical system, optical signal channels are generated, multiplexed to form an optical signal comprised of the individual optical signal channels, transmitted over a single wave-guide, and de-multiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM and DWDM approaches in long distance optical systems.
Crucial to providing sufficient bandwidth for WDM and DWDM, while at the same time avoiding bottlenecks, is the ability to assign and re-assign wavelengths as needed throughout the network and providing the bandwidth when and where needed. Allowing more flexibility in the way fiber capacity is provisioned is the driving force behind the requirements of next generation optical networks. Future network capacity needs will probably require a multi fold scalability beyond a network's initial installed capacity and also a rapid service activation to allow high capacity links to be deployed as needed.
Such requirements are best met by tunable lasers that can be tuned over a wide range of wavelengths and switched at nanosecond speeds. A number of schemes have been proposed and studied to obtain frequency tuning of semiconductor lasers. These methods have typically relied on tuning the index of refraction of the optical cavity. The resulting tunable range is, however, restricted to approximately 10 nm.
In addition, the bulk of the tuning schemes have been attempted with edge emitting laser structures. Unlike vertical cavity surface emitting lasers (VCSEL), these structures are not single mode and consequently the use of distributed Bragg reflectors or distributed feedback, both of which are difficult to fabricate, are required to select a single mode.
Interferometric techniques that rely on variable selection of different Fabry-Perot modes for tuning from a comb of modes have also been proposed. Among these are asymmetric y-branch couplers and vertical cavity filters. These methods produce tuning ranges of up to 100 nm, but are, however, restricted to discrete tuning only and are potentially unstable between the tuning steps.
Most of the above mentioned techniques are polarization sensitive and therefore cannot be readily adopted to optical communications systems, which need to be robust and inexpensive and consequently insensitive to beam polarization.
A critical and costly problem in all WDM and DWDM is created by the need for exact wavelength registration between transmitters and receivers. A tunable receiver capable of locking to the incoming signal over a range of wavelengths variation would relax the extremely stringent wavelength registration problem. The tunability requirement can best be met by proper VCSEL utilization. VCSELs possess desirable qualities for telecommunications: circular mode profile, single mode operation, surface mode operation and compact size. Complete description of the VCSEL device and its operation can be found in the issued U.S. Pat. Nos. 5,629,951 and 5,771,253 both of which are incorporated herein by reference.
The existing VCSEL technology cantilever apparatus is shown in FIG. 1. The cantilever apparatus has a cantilever structure consisting of a base, an arm and an active head. The arm deflects towards the substrate as a function of the voltage applied.
The arm, however, has built in stresses due to the materials it is constructed of and is supported at the base only and. This one point support combined with the built in stresses of the device through continual use causes the arm to droop and change its vertical position relative to the substrate. In more extreme cases the arm will adhere to the substrate. The change in the vertical position will in turn change the length of the Fabry-Perot cavity and consequently the resonant wavelength of the device. The change in the device resonant wavelength will in the end disrupt the communications system employing the device and causing it to drop the affected channel and search for another. The disruption will be enhanced in the systems using multiple VCSEL's as the droop and the subsequent resonant wavelength shift will vary from device to device. Additionally, the cantilever arm of the present device has a tendency to tilt. The tilt in turn causes loss in the device intensity. It may be possible to reduce the droop or the tilt by making the cantilever arm shorter. This approach would, however, require higher tuning voltages in order to obtain the same device performance. Additional approaches include better stress control in the cantilever arm or implementing a stop on the cantilever arm that would limit the droop and prevent the arm from contacting the substrate. Additionally, in the course of device manufacturing, upon completion of an etch process, the cantilever arm in the existing structure tends to adhere to the substrate and can only be separated by an additional drying process step. The additional process step increases the device cost and in some cases does not successfully free the arm form the substrate.
VCSEL of this invention is a bridge arm apparatus based on the principle of an electrostatic force pulling on a simple bridge arm as shown in FIG. 2. The device so formed is capable of continuously tuning the resonant frequency of the Fabry-Perot cavity over a wide range of wavelengths. The resonant cavity is formed between two distributed Bragg reflector (DBR) mirrors. The top reflector is composed of a movable top DBR supported in a bridge arm, a variable thickness air spacer layer and a fixed DBR. The bottom reflector is fixed in the substrate. By applying a tuning voltage to create electrostatic attraction, the bridge arm may be deformed towards the substrate, thereby changing the thickness of the air spacer layer and consequently the resonant wavelength of the Fabry-Perot cavity. A precise control of substrate to bridge arm distance is necessary in order to maintain the desired wavelength and meet the wavelength stability requirements.
Unfortunately, the bridge VCSEL structure is inherently large and has high parasitic capacitance. This in turn adversely affects high frequency operation of a VCSEL. A performance of a typical VCSEL as a function of frequency is shown in FIG. 5. FIG. 6 shows effects of increased capacitance on VCEL performance. It is clear from FIG. 6 that high capacitance adversely effects the high frequency performance of a VSCEL and consequently its data transmission rates. In this case the high capacitance is due primarily to the p contact bonding pad of the laser and the associated long leads as shown in FIGS. 3 and 3A.
For the reasons there is a need for a device structure that reduces capacitance to the levels necessary to achieve the appropriate high frequency, performance. The invention disclosed herein meets such need.